Development of a high-performance catalyst capable to accelerate steam-oxidative conversion of hydrocarbon mixtures (Table 1) without their preliminary separation is not only an important scientific task but also is of large economic significance. Urgency of this issue is determined by the fact that only in the Russian Federation due to the lack of necessary infrastructure there are tremendous amounts (more than 50 billion m3/year) of associated petroleum gas (APG) flared on oil production sites incurring serious economic losses and posing an environmental threat. Availability of efficient catalytic systems will allow to develop compact mobile plants which capacity will be commensurate to flow rate of oil wells in operation that will pave the way to practical APG utilization in remote regions.
TABLE 1Average Composition (% wt.) of APG from Different Oil Fields [SolovyanovA. A., Andreeva N. N., Kryukov V. A., Lyats K. G. Associated Petroleum Gas UtilizationStrategy in Russian Federation, Moscow: ZAO “Newspaper “Quarum” Editorial Office”,2008, 320 pp.].FieldCO2N2MethaneEthanePropanei-Butanen-Butanei-Pentanen-PentaneDanilovskoye1.072.1384.182.384.283.55C5-1.44, C6-10-1.59Barsukovskoye0.961.8080.785.814.272.042.001.160.65Samotlorskoye0.591.4860.644.1313.054.048.62.522.65Varyeganskoye0.691.5159.338.3113.514.056.652.21.8Tarasovskoye0.481.4754.1612.516.444.26.391.981.58Sovetskoye1.021.5351.895.2915.575.0210.332.993.26Uzenskoye—2.3050.2020.216.80—7.7—3.0Aganskoye0.51.5346.946.8917.374.4710.843.363.88Romashkinskoye1.58.038.8019.117.80—8.0—6.8Bavlinskoye0.48.435.0020.719.90—9.8—5.8
Difficulties of APG processing into synthesis gas are to a wide extent determined by the fact that it contains hydrocarbons with significantly different reactivity. For example, in n-alkane C1-C7 series the values of their formation constants decrease by many orders (Table 2) providing evidence of their stability reduction and carbon formation probability.
TABLE 2Values lgK*) of lower n-alkane formation at 1000° KMethaneEthanePropaneButanePentaneHexaneHeptane−1.0−5.71−9.98−14.10−18.40−22.64−26.89*)K = [CnH(2n+2)]/[H2](n+1)
This leads to intensive carbon deposit on catalyst surface and its fast deactivation.
Complexity of this problem solution is determined by the fact that catalysts for hydrocarbon conversion into synthesis gas and their cracking contain the same active components—group VIII metals. The most common approaches to reduction of carbon formation comprise optimization of process conditions (selection of a temperature regime, pressure, time of contact, combination of different oxidizers). Important results were obtained in the course of the development of new catalytic systems.
To increase the rate of carbon elimination from catalyst surfaces there are components introduced into their composition that are highly active in oxidation reactions. Cerium oxide allows to achieve the maximum effect that especially strongly shows itself in mixed oxides, for example, in Ce—Zr—O. Dispersiveness of an active metal in a catalyst is one of the factors that have an effect on carbon deposition because this process appears to be a structure-sensitive one. Carbon accumulation occurs with the participation of large enough metal particles, which size corresponds to a diameter of forming carbon nanotubes. The use of carriers capable of strong interaction with an active phase allows to prevent metal cluster agglomeration. The carbon formation rate depends on the active metal nature. The presence of platinum-group metals in a catalyst significantly decelerate this process.
Authors [Hotz N.; et al, AIChE Journal, 55(7), 1849-1859, 2009] suggested a sol-gel method for porous ceramic catalyst preparation by in-situ application of nanosized particles of Rh/Ce—Zr—O. This system displayed high thermal and catalytic stability in butane oxidative conversion into synthesis gas. The positive effect of cerium oxide as well appears in the case of Pt/CeO2/Al2O3 and Pt/CeZrO2/Al2O3 systems when CeO2 was introduced into a carrier [F. A. Silva et al, Appl. Catal. A: General 335 (2008) 145-152].
Besides the questions connected with catalyst chemical composition, the state and form of a catalyst is another very important problem. Variation of these parameters paves the way to the result-oriented adjustment of mass- and heat-transfer that is necessary for optimization of synthesis gas production technology. In this respect, porous monolythic catalysts on the base of heat-resistant materials (ceramics, metal alloys and their combinations) possessing low aerodynamic resistance appears to be very promising.
Utilization of metal carriers like foamed materials, lattices, perforated or corrugated foil (RU2204434, RU2248932, RU2268087, RU2292237, RU2320408) prevent catalyst local overheating due to their high thermal conductivity that prolongs the service life of a catalyst.
The process of preparation of catalysts deposited on metals with large surface area usually includes carrier preliminary oxidative treatment that significantly increases their adhesive properties maintaining stability of such systems. An additional operation for composite material efficiency improvement is the application on a metal (alloy) calcinated in an oxidative atmosphere of a primer agent, for example, pseudoboehmite, with active components introduced in its layer.
For instance, in one of the works (Bobrova L., Vernikovskaya N., Sadykov V.//Catal. Today. 2009. V. 144. P. 185) there was suggested a catalyst LaCeZrOx (5.3 wt. %)//LaNiPt (2 wt. %) on a fechral lattice, made from wire with the diameter of 2 mm. A method of its preparation included sputtering of a corundum layer on a lattice followed by deposition of γ-Al2O3 (3.6 wt. %) from a corresponding suspension. An active phase was formed by coating with LaCeZrOx suspension and impregnation with solutions of La, Ni and Pt compounds.
The homogeneity of catalyst active particles distribution on a carrier is achieved with the help of different methods. For example, in a catalyst preparation method according to U.S. Pat. No. 6,103,660, published Aug. 15, 2000, the slow homogeneous deposition of active component precursor particles on carrier particles is achieved: an active component precursor solution is introduced into a carrier particle suspension by capillary injection while continuously stirring. γ-Al2O3 or mixture of stabilized by lanthanum γ-Al2O3 with mixed Ce/Zr oxides with Ce, Zr, Ba acetates deposited on them is used as a carrier.
According to patent EP1759764, published Mar. 7, 2007, a hydrocarbon decomposition catalyst contains active metal particles (noble metals as well as Cr, Mn, Ti, Cu, Co, V and others, 0.025-10% wt. of a catalyst) of size 0.5-50 nm deposited on particles of a calcinated carrier of size 0.05-0.4 μm by any conventional method (precipitation, impregnation, equilibrium adsorption etc.). The main components of a carrier are Mg, Al, Ni (0.1-40% wt. of a catalyst), Si (0.001-20% wt. of a catalyst) in the form of mixed oxides. A carrier is obtained by thermal decomposition of a hydroxide mixture that are formed in alkaline environment from water-soluble salts and oxides (Si—from sodium silicate). The size of nickel particles in a catalyst may be of 1-20 nm.
In patent application US20120258857, published Oct. 11, 2012, there is described a method for obtaining a catalyst for autothermal reforming that appears to consist of magnesium, nickel and aluminium mixed oxides of size 40-300 nm that includes sol-gel synthesis of Mg, Ni and Al layered hydroxide precursor from salts of corresponding metals, its drying, at least partial decomposition at a temperature of 500-600° C. and reduction in H2—N2 environment at a temperature of 450-700° C. with obtaining nanosized particles. This catalyst is distinguished by low carbon formation rate and high activity.
From patent RU 2320408, published Mar. 27, 2008, and patent RU 2356628, published May 27, 2009, there is known a method for preparation of a catalyst that appears to be a heat-resistant armored carrier on which with the help of impregnation followed by heat treatment are deposited barium, manganese and cobalt mixed oxides. Mixed oxides consist of coarse-grain agglomerates of a few micrometer size and primary particles of 100-200 nm. A carrier—is a netted material of X23IO5T grade (fechral). The optimum conditions for the catalyst operation are: the O2/carbon ratio=0.5-0.6, H2O/carbon ratio=1.5-1.7, residence time 0.3-0.4 sec., temperature 800-950° C. The reaction products contain, %% vol.: hydrogen—32, methane—1, carbon dioxide—12, carbon monoxide—11, nitrogen—44. Changing of the water/carbon ratio within the range of 1.2-2.2 yields synthesis gas with the H2/CO ratio=2.3-3.65. This catalyst is resistant to carbon formation, at least during 100 hours of testing.
The closest to the claimed by us catalyst is a catalyst from U.S. Pat. No. 5,130,114, published Jul. 14, 1992 (prototype), for hydrocarbon steam reforming that incorporates a carrier—zirconium oxide, main active component—Rh and/or Ru and cocatalyst—at least one element from the group of Ni, Cr, Mg, Ca, Y and other rare earth elements. Catalyst high activity and low carbon formation rate is connected with the properties of a zirconium oxide as a carrier that can be used in a mixture with other carriers—SiO2, Al2O3, zeolite. A porous carrier may be deposited on a metal bed.